$ \left(\dfrac{64}{49}\right)^{-\frac{1}{2}}$
Answer: $= \left(\dfrac{49}{64}\right)^{\frac{1}{2}}$ Figure out what goes in the blank: $\Big(? \Big)^{2}=\dfrac{49}{64}$ Figure out what goes in the blank: $\Big({\dfrac{7}{8}}\Big)^{2}=\dfrac{49}{64}$ So $\left(\dfrac{64}{49}\right)^{-\frac{1}{2}}=\left(\dfrac{49}{64}\right)^{\frac{1}{2}}=\dfrac{7}{8}$